1. Field of the Invention
The present invention relates, generally to an automatic transmission having a pressure regulator and, more specifically, to an automatic transmission having a pressure regulator with flow force compensation.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first and oldest type is the manually operated transmission. These transmissions include a foot-operated start-up or launch clutch that engages and disengages the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever, and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. One disadvantage of the manual transmission is that there is an interruption in the drive connection during gear shifting. This results in losses in efficiency. In addition, there is a great deal of physical interaction required on the part of the operator to shift gears in a vehicle that employs a manual transmission.
The second and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. Automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween. In addition, automatic transmissions are typically more mechanically complex and therefore more expensive than manual transmissions.
For example, torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input>than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional “manual” and “automatic” transmissions have evolved. Most recently, “automated” variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated, or automatic, manual transmissions (AMTs) typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. The development of AMTs has provided a viable and improved means of power transmission for motor vehicles over the conventional automatic transmissions having a torque converter. However, even with the inherent improvements of these newer automated transmissions, they still have an operative power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a distinct shift feel that is generally associated with manual transmissions and may considered undesirable in certain operating environments.
To eliminate the power interruption in AMTs, other automated manual type transmissions have been developed that can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 4, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions (DCTs). The dual clutch structure is most often coaxially and cooperatively configured to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial, but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two dry disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission requires a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear selection and gear engagement in either an AMT or a DCT is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. The majority of the newer AMT and DCT designs employ 6 forward gears and a reverse gear, which provides greater efficiency and fuel economy by having closer ratio gear sets than previous designs.
While these new types of automatic transmissions have overcome several drawbacks associated with conventional transmissions, it has been found that controlling and regulating these automatically actuated transmissions to achieve the desired vehicle occupant comfort goals in an efficient and cost effective manner is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently.
Furthermore, since the control of these types of automatic transmissions is carried out by hydraulically actuating the various components within the transmission, it is important to provide a stable hydraulic supply pressure. Since hydraulically actuated devices respond in a predetermined and a precise manner for the given pressure supplied to actuate them, inaccurate control of the hydraulic supply pressure causes inaccurate operation and control of a AMT or DCT transmission. Up to this point, establishing and maintaining a stable hydraulic supply pressure in these newer types of automatic transmissions has proven problematic. As previously mentioned, a pump is employed to provide pressurized hydraulic fluid for the control and actuation of the transmission. In addition, the clutches and gear assemblies are lubricated and cooled by a secondary flow of hydraulic fluid. Typically, the pump is mechanically driven by a power take-off from the engine. Thus, the hydraulic pressure delivered from the pump increases as the pump speed increases in response to an increase in engine speed.
To address the changes in the hydraulic pressure delivered by the pump as engine speed changes, the hydraulic supply circuits of conventional dual clutch transmissions include a pressure regulator. More specifically, a pressure regulator is employed to establish and maintain a specific predetermined pressure in the hydraulic supply line. The pressure regulator includes a valve member slideably disposed within a valve body that moves back and forth over the various ports in the valve body to direct and control the fluid flow between the ports.
Since the pump is sized to provide the necessary pressure at idle and provides increased pressure as the engine speed increases, the pressure regulator is typically designed to dump, or bleed off the excessive flow to the return, or suction side of the pump. This action provides, at best, a rudimentary regulation of gross variations in pressure. However, the conventional regulation approaches fail to properly account for various flow effects of the hydraulic fluid within the hydraulic circuit and do not provide the precise and stable hydraulic supply pressure that is necessary to ensure accurate control over the AMT or DCT transmission. More specifically, to provide a stable regulated “line” pressure, the pressure regulator must be responsive to changes in the flow forces that occur within the regulator due to changes in the hydraulic flow in the line pressure side and the return, or suction side of the regulator.
The flow force is the relative force of the hydraulic fluid that acts upon the lands of the regulator valve member as the fluid moves through the pressure regulator. Flow forces are considered to be either steady state or transient. Steady state flow force is the force of the hydraulic fluid upon the valve member of the regulator that results from fluid accelerating property of the orifice formed within the regulator between the regulating valve member and the inlet and outlet ports. The steady state flow force is directly proportional to the pressure drop through the regulator and the area of the formed orifice. The steady state flow force always acts in a direction to close the regulator. Steady state flow forces relate to a steady state of the regulating valve member due to relatively constant flow conditions.
Transient flow force is the force in the hydraulic fluid that occurs when the valve member is moved and is due to the change of speed of the fluid moving through the valve as the size of the regulating area within the valve changes. The magnitude of transient flow force is proportional to the velocity of the movement of the valve member and pressure changes. The direction of the transient flow force depends on change to the flow. Transient flow forces relate to the regulating movements of the valve member.
The effects of these flow forces upon the pressures regulator are manifest as the fluid flow moves through the valve body of the regulator. As the hydraulic fluid moves through the regulator the inherent flow forces act against the physical surfaces of the valve member, the applied force can physically effect the position of the valve member in the valve body causing it to move and generate instability in the pressure regulator. For example, an increase in fluid flow from the pump may act upon the valve member surfaces of the pressure regulator forcing it open further, or an increase in pump suction may cause the regulator to move in an uncontrolled manner. The forced movement of the pressure regulator valve member by the flow forces causes instability in the line pressure and causes further variations in the flow as the regulator tries to correct. The effects of the flow forces upon the valve member of the pressure regulator may vary depending upon how the ports and the valve member of the pressure regulator are designed and how the pressure regulator is operatively placed in the hydraulic circuit.
Thus, while the current pressure regulators have generally worked for the intended purpose of gross regulation of line pressure to a relative range, they are still susceptible to flow forces fluctuations causing inaccurate hydraulic control of the transmission. Specifically, the structural configuration of a conventional transmission pressure regulator is one of two known types, neither of which is without certain drawbacks. These two types of pressure regulator configurations are based upon their different approaches to how the pressure is physically regulated or “metered.” One approach relates to a pressure regulator valve member and port interaction that is known as a “meter-in” configuration, in which the valve member of the regulator is designed to move across and regulate (i.e. meter) the line pressure on its line (inlet) port with the return or suction port of the regulator open and unrestricted. A meter-in configuration provides good control over the steady state flow but is generally unstable in regulating transient flow force. The other pressure regulator design approach is known as a “meter-out” configuration. With a meter-out configuration, the valve member of the regulator is designed to move across and regulate (i.e. meter) the line pressure on the suction (outlet) port with the line inlet port of the regulator open and unrestricted. A meter-out configuration provides good control during transient flow force conditions, but offers less stable control of the steady state flow force. The lack of valve stability in either of these pressure regulator configurations introduces line pressure fluctuations and subsequent inaccurate actuation and control of the dual clutch transmission. The inefficiencies and inaccuracies in hydraulic control of the dual clutch transmission that are attributable to the pressure regulator are distinct and produce quantifiable losses of vehicle output power and fuel economy.
In an AMT or DCT transmission, the pressure regulator is most often subject to steady state flow forces, while transient flow forces occur only during certain operational periods. Therefore, even with the above-mentioned instabilities, the meter-in configuration is the most common design type employed in conventional dual clutch transmissions. Attempts have been made to compensate for the transient flow force effects in meter-in configuration regulators. However, these attempts have been largely unsuccessful and have only made compensations for transient flow force effects at the expense of introducing instabilities in steady state flow force control. Thus, the conventional approaches employed with hydraulic pressure regulators in an AMT or DCT transmission remain inefficient and susceptible to fluctuations and inaccurate control of the line pressure causing inaccurate hydraulic control of the dual clutch transmission. Accordingly, there remains a need in the related art for an automatic transmission having a pressure regulator with flow force compensation that provides stable line pressure for both steady state flow and transient flow conditions.